Resonance-Raman-scattering spectroscopy for the halogen-molecular-ion centers in alkali halides

We have measured the Raman scattering in resonance with the bonding-to-antibonding transitions of two types of dihalogen-molecular-ion centers in alkali halides, the VK and H centers; the molecular ion occupies two adjacent halogen sites in the former and a single halogen site in the latter. It is found that the stretching-vibration frequency of the molecular ion is higher by more than 30% for the H center than for the VK center. The hardening is ascribed to the bond tightening due to the Madelung potential. Although the Raman line due to the stretching vibration consists of a single line in most alkali halides, two closely lying lines are observed for the H centers in alkali halides with small alkali-metal to halogen radius ratio and also for the VK center in NaCl. The paired-line structure is ascribed to the coupling of the stretching vibration of the halogen molecular ion with the lattice.